1. Field of the Invention
The present invention relates to a piezoelectric transducer, in particular, it relates to an improvement on the type of piezoelectric transducer commonly used for telephones, intercom systems and the like.
2. Discussion of Background
In piezoelectric transducers of this type, there is normally a considerable difference between the resonance frequency of the piezoelectric diaphragm and the cavity resonance frequency, resulting in poor frequency characteristics between the two resonance frequencies. As a means to solve this problem, a technology that uses a fabric damper is known in the prior art. In this technology, an appropriate degree of acoustic resistance is created by the fabric damper to flatten the frequency characteristics and to improve the frequency band width. This technology was disclosed in, for example, Japanese Laid Open Utility Model Publication 17899/1988. The fabric damper is usually made of a cloth with several hundred openings/inch and with this, the acoustic resistance requirement for flattening the frequency characteristics is ensured. The fabric damper is bonded onto the case that supports the piezoelectric diaphragm with an adhesive. The area in which the fabric damper is to be bonded has a damping hole and the entire surface of the fabric damper, excluding the area that corresponds to the damping hole, is adhered to the case. Such technology is disclosed in, for example, the aforementioned Japanese Laid Open Utility Model Publication No. 17899/1988.
Another reference on the prior art is Japanese Laid Open Utility Model Publication No. 68298/1988. This piezoelectric transducer is provided with a ring-like end surface on the circumference of the inner space of the case. The circumference of the piezoelectric diaphragm is secured on to the end surface of the case with an adhesive and a enclosure is formed by the inner space on the adhered surface-side. Lead wires are lead out to the outside from the enclosure. The lead wires are lead out to the outside of the case via a passage provided through part of the case. In order to maintain an air tight seal within the enclosed portion, this passage is sealed with a material such as silicon resin. The above mentioned piezoelectric transducer is disclosed in a receiver and transmitter described in Japanese Laid Open Utility Model Publication No. 135851/1989. The two piezoelectric transducers, one for transmission and the other for reception, are separately arranged inside the handset main unit so that they correspond with the receiving and transmitter sections. In order to maintain an air tight seal between the space leading to the piezoelectric transducers and the inner space of the handset main unit, a packing member is provided between the two spaces. The packing member is provided as an independent component, separate from the piezoelectric transducers and the handset main unit and it is secured between the piezoelectric transducers and the handset main unit by the assembly clamping force of the piezoelectric transducers against the handset main unit.
As described above, since the fabric damper is made of a meshed cloth, when it is bonded with an adhesive, the adhesive tends to seep all over the surface of the bonded area surrounding the damping hole, resulting in deterioration of the acoustic characteristics of the fabric damper and consequently causing a damping failure. The foregoing represents a problem with the above-mentioned prior art technology. The prior art technology described above does not disclose any means for solving this problem of damping failure due to seeping adhesive.
Also, since the lead wires connected to the piezoelectric element are lead out to the outside of the case via a passage provided in part of the case, the sealing agent for maintaining the air tight seal of the enclosure sometimes runs out via the passage to the surface of the outer circumference of the case, resulting in a defective product.
Moreover, since, in the prior art receiver and transmitter, the packing member is provided as an independent component, separate from the piezoelectric transducer and the handset main unit, and is secured between the piezoelectric transducers and the handset main unit by the assembly clamping force of the piezoelectric transducers against the handset main unit, the packing member is liable to shift at the time of assembly causing misalignment and resulting in loss of the air tight seal. Therefore, extra care must be taken so that misalignment will not occur. This means reduced efficiency in assembly work. Furthermore, the misalignment problem can never totally solved, even when maximum care is taken in assembly. Also, as the packing member is an independent component, separate from the handset main unit and the piezoelectric transducers, the number of components and assembly steps is greater so production costs are increased.